The present invention relates to a system and method for establishing communication and cooperation between two nominally identical electronic devices to enable either device to be operated as an active device and the other to serve as a backup, or standby, device.
In many industrial systems, it is considered advantageous, if not essential, to provide redundancy with respect to at least certain devices. This occurs, for example, in the case of computerized industrial systems employing devices known as progammable controllers, where it is desired, at certain points in the system, to provide two structurally and functionally identical controllers, with one functioning as the active device in the system and the other being a standby device which, in the event of a failure in the active device, can take over its function.
Programmable controllers belong to the class of relatively "intelligent" devices which are provided with their own microprocessor and memories and which are programmed to carry out preselected operating sequences in response to instructions from a central control unit and in response to data derived from the operations being performed. Thus, the operation of such controllers involves a continuous exchange of data and instructions with other devices in the system, and the operation performed by such a controller at any given time is dependent on the data and instructions previously supplied thereto.
Heretofore, in situations where a standby device was provided, it has been the usual practice to interconnect the active and standby devices by a separate device which controls the relationship therebetween and to cause the standby device to remain completely idle as long as the active device is functioning properly. If a failure should occur in the active device, it is necessary to transmit a large mass of data to the backup device before it can be switched to take over the function of the active device. This requires a relatively long processing time during which neither of the two process controllers is active.